Remove_Nth_Node_From_End_of_List.py

"""
Given a linked list, remove the nth node from the end of list and return its head.

For example,

   Given linked list: 1->2->3->4->5, and n = 2.

   After removing the second node from the end, the linked list becomes 1->2->3->5.
Note:
Given n will always be valid.
Try to do this in one pass.
"""

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    # @return a ListNode
    def removeNthFromEnd(self, head, n):
        fast = head
        dummy = ListNode(0)
        dummy.next = head
        while n > 0:
            fast = fast.next
            n -= 1
        slow = dummy
        while fast is not None:
            fast = fast.next
            slow = slow.next
        slow.next = slow.next.next
        return dummy.next